1. Field of the Invention
The present invention relates to two-stage, three-terminal thermionic/thermoelectric coolers.
2. Description of the Related Art
Temperature stabilization and control for laser sources, switching/routing elements, and detectors in high speed and wavelength division multiplexed optical telecommunication systems is typically accomplished with thermoelectric (TE) coolers. Since optoelectronic devices are not easily integrated with TE coolers, the cost of packaging is high. In addition, a TE cooler usually limits the reliability and lifetime of a packaged module.
An alternative to conventional TE coolers is heterostructure integrated thermionic (HIT) coolers. These thin film coolers use the selective emission of hot electrons over a heterostructure barrier layer from cathode to anode, resulting in evaporative cooling. The current state of the art for HIT coolers is currently on the order of several degrees cooling across films on the order of one micrometer thick at room temperature. Cooling on the order of several degrees over one-to-two micron thick barriers has been demonstrated in conventional material systems, such as InGaAsP and SiGe, which corresponds to cooling power densities of several hundred Watts/cm2. Single stage thermoelectric coolers are capable of cooling by roughly 70xc2x0 C. and maximum cooling power densities of approximately 10 Watts/cm2 near room temperature.
A limitation in these thin film devices is the thermal resistance of the substrate on which the epitaxial films are grown. This thermal resistance between the hot side of the cooler and the heat sink can cause much of the heat to flow back to the cold side of the cooler. Several methods for transferring the epitaxial films to surrogate substrates with high thermal conductivity are possible, but they complicate considerably the processing and packaging.
A simpler way of effectively reducing the substrate thermal resistance is to use the substrate itself as a thermoelectric cooler. This concept has been successfully employed in the tuning of in-plane and vertical cavity lasers by changing the current through metal-substrate contact.
The present invention uses thermionic (TI) and thermoelectric (TE) coolers in a two-stage, three-terminal configuration, providing a novel device geometry. The most general use of the cooler of the present invention, also known as a thermionic/thermoelectric (TI/TE) cooler, is for solid state cooling of devices, components, and systems. Since the cooler of the present invention can be fabricated using many different material systems, it lends itself well to integration with optoelectronics and microelectronics.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a multi-stage cooler formed from monolithically integrated thermionic and thermoelectric coolers, wherein the thermionic and thermoelectric coolers each have a separate electrical connection and a common ground, thereby forming a three terminal device. The thermionic cooler is comprised of a superlattice barrier surrounded by cathode and anode layers grown onto an appropriate substrate, one or more metal contacts with a finite surface area deposited on top of the cathode layer, and one or more mesas of different areas formed by etching around the contacts to the anode layer. The thermoelectric cooler is defined by metal contacts deposited on the anode layer or the substrate itself. A backside metal is deposited on the substrate for connecting to the common ground.